The present invention generally relates to a semiconductor laser control apparatus, and in particular, to an apparatus for controlling the intensity of a beam emitted from a semiconductor laser built in a laser printer, a facsimile machine, a digital copier or the like.
Conventionally, an automatic output control circuit for use in a semiconductor laser uses an 8-bit digital-to-analog converter, by which a resolution of 1/256 is obtainable. Hereafter, a digital-to-analog converter is simply referred to as a D/A converter. However, there is a great variation in the output density of a semiconductor laser due to a quantization error in the 8-bit D/A converter. For example, when a current for driving the semiconductor laser ranges from 20 mA to 120 mA, it is difficult to control the output of the semiconductor laser with sufficient accuracy. From this viewpoint, a semiconductor laser control apparatus is known in which a 12-bit D/A converter is employed in order to reduce the quantization error. However, the use of the 12-bit D/A converter requires an extremely large amount of time when the output of the semiconductor laser is initially set. Additionally, commercialized 12-bit D/A converters are more expensive than 8-bit D/A converters.
FIG. 1 is a block diagram of a conventional semiconductor laser control apparatus. Referring to FIG. 1, a semiconductor laser (hereinafter referred to as a laser diode) 10 is driven by a drive signal supplied from a semiconductor laser drive circuit (hereafter simply referred to an LD drive circuit) 11. The laser diode 10 emits laser beams forward and backward. The light beam emitted forward is irradiated on a recording medium (not shown), and the light beam emitted backward is projected onto a photosensor 12. The photosensor 12 produces an output current proportional to the intensity of the laser beam emitted from the laser diode 10. The output current is converted into a voltage signal V.sub.M by an amplifier 13. The converted voltage is compared with a reference voltage V.sub.ref1 by a comparator 14. The comparator outputs a signal switchable between low and high levels depending on the comparison result. The output signal derived from the comparator 14 is used to control the count mode of an up/down counter 15. Hereinafter, the up/down counter is simply referred to as a counter. When the output voltage V.sub.M is lower than the reference voltage V.sub.ref1, or in other words, the light intensity of the laser diode 10 does not reach a reference value P (FIG. 2), the output signal of the comparator 14 is kept at the high level, so that the counter 15 is set to an up count mode in which the counter 15 functions as an up counter. On the other hand, when the output voltage V.sub.M is higher than the reference voltage V.sub.ref1, the counter 15 is set to a down count mode in which the counter 15 functions as a down counter.
A flip-flop (hereafter simply referred to as an F/F) 16 is set by the application of a power set signal S1, which is supplied at the commencement of a standby mode. Then the F/F 16 releases the counter from a disabled state. Thereby, the counter 15 counts a clock pulse generated by a clock pulse generator 17 in accordance with the output signal of the comparator 14 applied to an up/down (U/D) terminal of the counter 15.
The counter 15 gradually increases or decreases the count value therein, depending to the variation in the intensity of the light emitted from the laser diode 10. The count value is converted into an analog signal by a D/A converter 18. A converted analog signal is supplied to the LD drive circuit 11, which changes the drive signal to be supplied to the laser diode 10. Thereby, the intensity of the light emitted from the laser diode 10 is controlled. An edge detection circuit 19 detects an edge of the rise (or fall) of the output signal of the comparator 14, and resets the F/F 16. Thereby, the counter 15 returns to the disable state.
The above-mentioned operation is illustrated in FIG. 2. At the time of the first power setting operation upon power ON, the light intensity of the laser diode 10 is increased up to the reference value P, and is kept thereat. Then, at the time of the power setting operation on and after the second time is carried out by starting the power setting control from the reference value P, and increases the light intensity to the reference value P.
However, the conventional structure has the disadvantages described below. As described previously, the intensity of the light emitted from the laser diode 10 is increased to the reference value P by the single D/A converter 18. Therefore, the light intensity varies greatly due to the quantization error which occurs in the D/A converter 18. Further, a smooth and rapid power setting control is not obtainable, particularly when the D/A converter 18 is constituted by the 12-bit D/A converter.